A sprawling metropolis of 100,000 people or more—perhaps the largest city in the world at the time—Teotihuacan left no records that epigraphers have been able to decipher. But its motives in dispatching Fire Is Born to the Maya region seem clear. Waka sat on a promontory overlooking a tributary of the San Pedro with a protected harbor, excellent for berthing large canoes. "It was a perfect staging area" for military action, notes Southern Methodist University archaeologist David Freidel, co-director of excavations at Waka. Which may be precisely what Fire Is Born had in mind.

Waka appears to have been key to the envoy's mission: to bring the entire central Petén into Teotihuacan's orbit, through persuasion if possible, force if necessary. His principal target was Tikal, a kingdom 50 miles (80 kilometers) east of Waka. Tikal was the most influential city-state in the central Petén. Bring Tikal into the fold, and the other cities would follow.

Fire Is Born's soldiers were probably shock troops, designed principally to display his bona fides and demonstrate good faith. He needed reinforcements, and he had come to Waka to get them. In return, he could offer the goodwill of his patron, a mysterious ruler known from inscriptions as Spear-thrower Owl, probably a highland king, perhaps even the lord of Teotihuacan.

Waka's ruler, Sun-faced Jaguar, apparently welcomed Fire Is Born. Based on hints in texts from Waka and other sources, Freidel, project co-director Héctor Escobedo, and epigrapher Stanley Guenter suggest that the two rulers cemented their alliance by building a fire shrine to house the sacred flame of Teotihuacan.

Along with moral support, Fire Is Born probably secured troops. His expeditionary force likely carried the spear-throwers and javelins typical of Teotihuacan and wore backshields covered with glittery pyrite, perhaps meant to dazzle the enemy when the soldiers spun around to hurl their weapons. Now warriors from the Petén, equipped with stone axes and short stabbing spears, swelled their ranks. As armor, many wore cotton vests stuffed with rock salt. Eleven hundred years later, the Spanish conquistadores shed their own metal armor in the sweltering rain forest in favor of these Maya "flak jackets."

The military expedition most likely set out for Tikal in war canoes, heading east, up the San Pedro River. Reaching the headwaters, the soldiers disembarked and marched either along the river or on the canyon rim overlooking it.

Garrisons probably dotted the route. News of the advancing column must have reached Tikal, and somewhere along the stretch of riverbank and roadway, perhaps at a break in the cliffs about 16 miles (26 kilometers) from the city, Tikal's army tried to stop Fire Is Born's advance. Inscribed slabs, called stelae, later erected at Tikal suggest that the defenders were routed. Fire Is Born's forces continued their march on the city. By January 16, 378—barely a week after his arrival in Waka—the conqueror was in Tikal.